Sanfilippo B syndrome is a devastating lysosomal storage disorder due to deficiency of the enzyme alpha-N- acetylglucosaminidase (NAGLU). Most therapeutic approaches that have been used or proposed for the lysosomal storage diseases that are due to missing enzymes involve replacing that enzyme into the deficient host in some way- either using wild-type donor stem cells (which provide the missing enzyme), gene therapy to deliver the gene that encodes the enzyme, or by recombinant enzyme administration. All of these depend on the special property of lysosomal enzymes that is made possible by the mannose 6-phosphate receptor system: lysosomal enzymes can be secreted by a cell, taken up by another cell via mannose 6-phosphate receptors, and shuttled to lysosomes where they can act to reduce stored substrate. Unfortunately for people affected with Sanfilippo B syndrome, NAGLU does not contain sufficient mannose 6-phosphate when expressed recombinantly, so that cellular uptake and lysosomal targeting never take place. We have generated a fusion construct of NAGLU and a portion of insulin-like growth factor 2 (IGF2). The natural scavenger receptor for IGF2 is the mannose 6-phosphate receptor (though it binds at a different site than does mannose 6- phosphate). Our preliminary in vitro and in vivo data show that recombinant human NAGLU-IGF2 is active, enters cells, and can reduce lysosomal storage with high efficiency. The next step, which this proposal seeks to address, is to determine whether recombinant human NAGLU-IGF2 will improve the Sanfilippo B phenotype, which is one of progressive cognitive deterioration. In this project, we propose to study this potential new therapy in vivo, to determine whether it improves neurobehavior, physical disease, and survival. The results will tell us whether recombinant NAGLU-IGF2 can serve as effective enzyme therapy for this devastating disease. Results may also have implications for gene and cell therapy approaches for Sanfilippo B, which might be made more effective with the use of IGF2-mediated lysosomal targeting.